A direct-alcohol fuel cell comprises a cathode, an electrolyte, and an anode. The cathode is fed an oxidizing agent, for example in the form of air, and the anode is fed an alcohol fuel, for example in the form of a methanol-water mixture.
In order to achieve high electric output, a plurality of fuel cells are generally electrically and mechanically connected to each other by connecting elements (bipolar plates). This creates electrically series-connected fuel cells that are stacked on top of each other, which are referred to as a fuel cell stack. They comprise the bipolar plates and electrode-electrolyte units.
In a direct-methanol fuel cell, the fuel is added in the form of a methanol-water mixture. The depleted, which is to say partially reacted, methanol-water mixture, is removed from the anode regions in order to recycle the fuel on the anode side. It is known that during the electrochemical reaction, not only protons, but also non-negligible quantities of water and alcohol, which in this case is methanol, always migrate through the membrane onto the cathode side. This unreacted methanol would usually arrive at the exhaust, together with the resulting water that is created at the cathode, by way of the cathode exhaust gas line, which of course should be prevented.
Thus, according to the state of the art, as an alternative, a catalytic burner is disposed downstream of a direct-alcohol fuel cell stack on the cathode exhaust gas side, the object of this burner being to burn the organic constituents from the cathode exhaust gas by way of the residual oxygen.
The disadvantage is that, for reasons having to do with energy, direct-methanol fuel cell stacks are nowadays operated at a temperature between 70 and 80° C. The air that exists as part of the process on the cathode side is generally saturated with water vapor. In addition, the oxygen that is used as an oxidizing agent is usually consumed during the electrochemical reaction in the fuel cells. Thus, depending on the operating parameters, the remaining oxygen content amounts to just a few percent, or even less, in the cathode exhaust gas. As a result, the residual oxygen content is hardly sufficient for the catalytic combustion.
As another alternative for reducing alcohol emissions from a direct-alcohol fuel cell, a catalytic reaction in the exhaust gas collector line, on the cathode side, has been proposed in the literature, as has a porous layer coated with a catalyst, which brings about the reaction of unreacted alcohol, inside a cathode region of each fuel cell.
Likewise known is the alternative of firstly directing the cathode exhaust gas flows, individually, or as a collective exhaust gas flow, to an anode region of an additional or identical fuel cell, and then electrochemically reacting the unreacted alcohol there. Accordingly, this configuration has only one common discharge port for the exhaust gas from the anode region.